The exploration and recovery of gas and oil requires drilling a borehole into the ground through the geological formations of interest. Drilling mud is used to lubricate and cool the drill bit, to assist in bringing the drill cuttings to the surface, to provide sufficient hydrostatic pressure within the well to prevent the bore collapsing under the forces experienced underground, and to prevent the influx of fluid from the formation while drilling. The drilling mud can be either aqueous based or oil or synthetic based.
After drilling the borehole a tubular casing is secured within the borehole with cement. This insures permanence of the borehole and prevents entry of formation fluid, other than from the formation which is being exploited. It is common procedure to pump the cement down the inside of the casing to the bottom and then up the outside of the casing, between the casing and the borehole wall, until it fills and lines the section of the annulus to be cemented. It is important that the cement lining be free of voids and the cement be well bonded to both the casing and borehole wall. If good bonding is not established between the cement, casing and borehole wall surfaces, serious problems can arise. In addition, the cement chemistry is very sensitive to oil and hydrophobic solids. The complete removal of these contaminants from the wellbore is therefore essential to a better control of the cement setting and flow properties.
To ensure good bonding between cement and the appropriate surfaces, it is necessary to remove substantially all traces of drilling mud on the casing and the borehole wall. Incomplete mud removal can leave a channel behind which could prevent total isolation of a production zone. The substantial or complete removal of drilling mud often proves extremely difficult and various methods and mechanisms have been devised in an attempt to achieve complete mud removal and to provide a complete cement lining, free of voids, between the casing and the borehole wall.
One method involves the use of spacer fluids, i.e., liquids that literally wash the mud off of the casing and borehole wall, ahead of the cement. Spacer fluids can be pumped so that they are positioned between the cement and the mud. Such fluids are sometimes referred to as “chemical washes” which are usually low-viscosity liquids containing surfactants and mud thinners, or diluents. Spacers can also be viscous, gel-like liquids that are primarily used to form a buffer between the cement and the mud. Typically, a spacer is characterized as a thickened composition that functions primarily as a fluid piston in displacing the mud. Frequently, spacers contain an appreciable amount of weighting materials, as well as fluid loss control agents. Chemical washes, on the other hand, are generally thin, or relatively non-viscous, fluids which are effective principally as a result of turbulence, dilution, and surfactant action on the mud and mud filter cake. Chemical washes may contain some solids to act as an abrasive, but the solids content is generally significantly lower than in spacers because chemical washes are typically too low in viscosity to have good solids carrying capacity.
While there are various chemical washes and spacer fluids in commercial practice today, there is a need for an improved composition and method for displacing drilling fluids and leaving the borehole surfaces clean for adequate bonding and setting of cement in the annular space defined by the casing and borehole wall.